Power assist steering systems for land vehicles that provide assistance during a steering maneuver are well known. Such systems typically utilize hydraulic power or electric power.
During operation of a vehicle at a relatively low speed, such as during dry surface parking, there is a relatively large resistance to turning movement of the steerable vehicle wheels. As vehicle speed increases, the resistance to turning movement of the steerable vehicle wheels decreases. To compensate for the decreasing steering resistance, some power assist steering systems provide one level of steering assist at low vehicle speeds and decreasing assist as vehicle speed increases. To accomplish such variable assist, power assist steering systems have been designed to monitor the vehicle speed and to control the power assist in response to the monitored vehicle speed.
One proposed hydraulic power assist steering system that controls power assist in response to vehicle speed is disclosed in U.S. Pat. No. 4,456,087 to Lang et al. The steering system includes an electromagnetic valve that is actuated by a microprocessor and a speed sensor which generates a signal indicative of the vehicle speed. The microprocessor includes a read only memory having stored tabulated data based on a functional relationship between the speed dependent input signal from the speed sensor and a desired fluid pressure signal control to be exercised by the electromagnetic actuated valve. The microprocessor controls the valve in response to vehicle speed and in accordance with the stored tabulated data.
In power assist steering systems that vary the power assist in response to vehicle speed, the power assist at any given vehicle speed is preset. Since satisfactory steering "feel" is a subjective interpretation, vehicle operators have varying opinions as to the power assist they would like at a given vehicle speed.